1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a driving circuit for a liquid crystal display device.
2. Description of the Related Art
In general, a liquid crystal display (LCD) device is formed by attaching a thin film transistor (TFT) array substrate and a color filter (CF) substrate together to face each other with a specified cell gap therebetween, and filling the cell gap with a liquid crystal material. A plurality of gate lines are arranged at regular intervals along a horizontal direction and a plurality of data lines are arranged at regular intervals along a vertical direction on the TFT array substrate to cross each other. Crossings of the data lines with the gate lines define pixel regions. Each pixel region includes a switching device and a pixel electrode. In addition, red, green and blue color filters corresponding to the pixel regions are formed on the CF substrate. A black matrix is formed in a mesh shape that encompasses an outer edge of the color filters. The black matrix prevents color interference of light passing through the pixel regions. Furthermore, a common electrode is formed on the CF substrate. The common electrode and the pixel electrode generate an electric field through the liquid crystal material.
Twisted nematic (TN) liquid crystal material is commonly used in LCD devices. In a TN liquid crystal display device, a vertical electric field drives the liquid crystal material. The vertical electric field is generated between the pixel electrode formed on the thin film transistor array substrate and the common electrode formed on the color filter substrate. Accordingly, light transmittance of the TN liquid crystal material changes according to a viewing angle of an observer. Especially, the light transmission is asymmetrically distributed with respect to a vertical viewing angle, generating a range in which an image is reversed vertically and causing a narrow viewing angle. As a result, the fabrication of a large area liquid crystal display panel is difficult.
In order to solve the above problem, an in-plane-switching (IPS) mode liquid-crystal display device has been suggested for driving the liquid crystal material with a horizontal electric field. The IPS LCD device may improve angular field characteristics, such as contrast, gray inversion, and color shift, thus providing a wide angular viewing field, in comparison to the LCD device in which the liquid crystal material is driven by a vertical electric field. Hence, the IPS LCD device is commonly used in large-size LCD devices.
FIG. 1A is a planar view illustrating a pixel of a related art in-plane switching mode liquid crystal display device. FIG. 1B is a cross sectional view of the in-plane switching mode liquid crystal device of FIG. 1A. Referring to FIGS. 1A and 1B, gate lines 1 and data lines 3 form a matrix on a first substrate 10, thus defining a pixel region. A thin film transistor 9 consisting of a gate electrode 1a, a semiconductor layer 5 and source/drain electrodes 2a and 2b is formed at a crossing of one of the gate lines 1 and one of the data lines 3. The gate electrode 1a is electrically connected to the gate line 1, and the source/drain electrodes 2a and 2b are electrically connected to the data line 3.
A common voltage line 4 is arranged parallel to the gate line 1 in the pixel region. At least one pair of a common electrode 6 and a pixel electrode 7 is arranged parallel to the data line 3 for applying an electric field to liquid crystal molecules. The common electrode 6 is formed simultaneously with the gate line 1 and connected to the common voltage line 4. The pixel electrode 7 is formed simultaneously with the source/drain electrodes 2a and 2b and connected to the drain electrode 2b of the thin film transistor 9. A passivation film 11 is formed on the entire surface of the substrate 10 including the source/drain electrodes 2a and 2b. A pixel electrode line 14 is formed to overlap the common line 4 and is connected to the pixel electrode 7, thus forming storage capacitors Cst.
A black matrix 21 for preventing light leakage to the thin film transistor 9, the gate line 1, the data line 3, and a color filter 23 for displaying a color image are formed on a second substrate 20. An overcoat film (not shown) for flattening the color filter 23 is formed thereon. Alignment films 12a and 12b are formed at facing surfaces of the first and second substrates 10 and 20. The alignment films 12a and 12b determine an initial alignment direction of liquid crystals.
A liquid crystal layer 13 is provided between the first and second substrates 10 and 20. The light transmittance of the liquid crystal layer 13 is controlled by a voltage applied between the common electrode 6 and the pixel electrode 7. The related art in-plane switching mode LCD device having the above-described structure can improve the viewing angle because the common electrodes 6 and the pixel electrodes 7 are disposed on the same plane and thus generate an in-plane electric field.
FIG. 2 is a schematic view of the pixel regions in the liquid crystal display device of FIG. 1. Referring to FIG. 2, the liquid crystal display device includes a plurality of data lines DL1 to DLm arranged at regular intervals in a vertical direction, a plurality of gate lines GL1 to GLn arranged at regular intervals in a horizontal direction, a plurality of pixels P1 formed by crossings of the data lines DL1 to DLm and the gate lines GL1 to GLn, and a plurality of common voltage lines VL1 corresponding to the gate lines GL1 to GLn and supplying a common voltage to the pixels P1.
The pixels P1 are electrically connected to the gate lines GL1 to GLn and the data lines DL1 to DLm. More specifically, the gate electrode of the thin film transistor T1 provided within each of the pixels P1 is connected to one of the gate lines GL1 to GLn, and the source electrode thereof is connected to one of the data lines DL1 to DLm. A liquid crystal capacitor Clc and a storage capacitor Cst are electrically connected in parallel between the drain electrode of the thin film transistor T1 and the common voltage line VL1. The common voltage line VL1 is commonly connected to each of the pixels P1, thereby supplying the same common voltage VCOM to each pixel P1.
The gate lines GL1 to GLn are sequentially activated by applying a scan signal from a gate driving unit (not shown). The scan signal is applied to the gate electrodes of a plurality of thin film transistors T1 connected to the corresponding gate lines GL1 to GLn, thereby turning on the thin film transistors T1. As stated above, the source electrodes of the thin film transistors T1 are connected to the data lines DL1 to DLm, and thus an image voltage applied to the data lines DL1 to DLm is provided to the source electrodes of the turned-on thin film transistors T1.
When an electric field is continuously supplied to the liquid crystal material, the liquid crystal material deteriorates, thereby causing afterimage distortions due to a DC voltage component. To eliminate the DC voltage component, and prevent deterioration of the liquid crystal material, a positive (+) voltage and a negative (−) voltage corresponding to the image information are alternately supplied as the common voltage. Such a driving method is commonly called an inversion driving method.
Several types of inversion driving methods have been proposed. In a frame inversion driving method, a polarity of the supplied image information is inverted for each frame period. In a line inversion driving method, the polarity of the supplied image information is inverted for each gate line. In a dot inversion driving method, the polarity of the supplied image information is inverted from one pixel to the adjacent one, and also inverted for each frame period.
FIG. 3A illustrates typical voltage waveforms corresponding to an image voltage and a common voltage in accordance with the line inversion method. FIG. 3B illustrates typical voltage waveforms of an image voltage and a common voltage in accordance with the dot inversion method. Referring to FIGS. 3A and 3B, a voltage difference between the supplied image voltage Vdata and the common voltage VCOM is set to 5V.
Referring to FIG. 3A, the common voltage VCOM applied through a common voltage line to each pixel P1 is shifted from a high voltage (5V) to a low voltage (0V) or from a low voltage (0V) to a high voltage (5V) at each horizontal period. The supplied image voltage Vdata is applied to a pixel at each horizontal period with a polarity opposite to that of the common voltage VCOM. Thus, even if the swing of the image voltage Vdata is reduced to a range between 0V to 5V, the voltage difference ΔV1 between the common voltage VCOM and the image voltage Vdata can be increased.
Referring to FIG. 3B, the common voltage VCOM applied to the common voltage line is a direct current voltage having the same level in each horizontal period. Thus, if the voltage level of the common voltage VCOM is fixed, the voltage difference ΔV2 between the common voltage VCOM and the image voltage Vdata can only be controlled by adjusting the image voltage Vdata. Thus, to set the voltage difference ΔV2 to a value of 5V as in the line inversion method, the image voltage Vdata has to be swung from 0V to 10V, which increases power consumption compared to the line inversion method. Hence, the line inversion method can reduce the power required for switching the image voltage Vdata compared to the dot inversion method. However, in the line inversion method, the common voltage VCOM has to be driven along with the image voltage Vdata. Especially, in the in-plane switching mode liquid crystal display device, a common voltage VCOM larger than that required by the related art TN liquid crystal display device has to be applied to drive the common voltage VCOM at a direct current.
In the twisted nematic liquid crystal display device, the common voltage applied to the dots of TFT array substrate drives a relatively low load because the common electrode is formed over the entire surface of a color filter substrate. In contrast, in the in-plane switching mode liquid crystal display device, both the common electrode and the pixel electrode are provided together within the pixel region. For example, the common electrode is usually formed in a long, narrow bar shape within the pixel region to increase the aperture ratio of the pixel. Thus, the common electrode in the IPS mode LCD can have a relatively high resistance. Accordingly, the common voltage applied from the driving circuit drives a large load. Hence, the common voltage waveforms applied to each pixel are delayed or distorted by the relatively large load.
FIG. 4 shows a typical common voltage waveform applied to pixels in a related art in-plane switching mode liquid crystal display device. Referring to FIG. 4, a dotted line represents a ideal waveform for a common voltage VCOM, and a solid line represents an actual waveform for the common voltage VCOM applied to the pixels of a related art IPS mode LCD. As can be inferred from FIG. 4, in the in-plane switching mode liquid crystal display device, the resistance of the common electrode provided in each pixel region is high, and the overall resistance of liquid crystal display is high, thus causing a time delay before the applied common voltage VCOM reaches a desired voltage level. The time delay before the common voltage VCOM can reach a desired voltage level causes the corresponding waveform to have a slowly rising curve shape rather than a square shape. Also, the common voltage VCOM might fall without ever reaching the desired level within the horizontal period.
When the common voltage VCOM cannot reach a desired voltage level, the image voltage has to be increased to form a desired voltage difference between the common electrode and the pixel electrode, thus increasing the required driving power. Further, when the common voltage VCOM cannot reach the desired level, the pixels are not charged with a sufficient voltage, thereby deteriorating the quality of the displayed image.
As discussed above, when the load provided by each of the pixels of the in-plane switching mode liquid crystal display device increases, in spite of using the line inversion method for reducing power, the benefits provided by the method are hampered, making it difficult to charge the pixels with sufficient electric charges, and, thereby, deteriorating the quality of the displayed image.